About 850,000 people joined the candlelight rally at Gwanghwamun and a public square near Seoul City Hall as of 6:30 p.m., according to organizers.
"Considering the specific objective of this rally, which is to convey the public voice to the president, the meaning Sajik street and Yulgok street hold is significantly different from what they meant for other rallies", the Seoul Administrative Court was quoted as saying.
South Korea urged protesters seeking scandal-hit President Park Geun-hye's resignation to show restraint at a mass march on her complex planned for the weekend. The placards read, "Step down Park Geun-hye".
She said anyone found to have done wrong would be punished, and that she was prepared to be investigated by prosecutors.
Choi is accused of advising Park on political appointments and policy decisions, and using her influence to force corporations to donate to the foundations that she works with. "Given that in the past, North Korea staged provocations during the period of the government transition in the U.S., we need to closely cooperate in advance to thoroughly deter possible North Korean provocations and respond sternly if provoked".
Protesters also marched on a road in front of the palace gate and near the Blue House, the mountainside presidential office and residence, carrying candles, blowing horns and banging drums, while shouting "Park Geun-hye, resign!"
Park's acknowledgement only served to fuel public anger and suspicion over the secret confidant, who apparently held no official government position.
The four leading South Korean opposition leaders have all called for President Park to step down, saying that she is not fit to manage the diplomatic shake-up that President-elect Trump is predicted to bring to the Korean Peninsula.
Officials said Abe and Trump also confirmed their resolve to cooperate in ensuring peace and stability in the Asia-Pacific region, but did not discuss the trans-Pacific trade pact and other contentious issues such as the cost of American troops in Japan.
South Korea's opposition party presidential hopeful also congratulated Trump on his victory, No Cut News reported.
On Friday, prosecutors summoned the chairman of steelmaker Posco over allegations that Ms Choi and her associates tried to forcibly take over the shares of an advertising company previously owned by the firm. Police arrested her earlier this month on charges of fraud and abuse of power. If she steps down before the end of it, an election must be held within 60 days. During the phone conversation, she asked him to visit South Korea and the U.S. President-elect gave a positive answer.
The embattled president made public apologies twice and accepted a coalition cabinet formed by the parliament, but people get infuriated further on growing views that Park has yet to sincerely confess to her sins.